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RFC788
SIMPLE MAIL TRANSFER PROTOCOL
Jonathan B. Postel
November 1981
Information Sciences Institute
University of Southern California
4676 Admiralty Way
Marina del Rey, California 90291
(213) 822-1511
RFC 788 November 1981
Simple Mail Transfer Protocol
TABLE OF CONTENTS
1. INTRODUCTION .................................................. 1
2. THE SMTP MODEL ................................................ 2
3. THE SMTP PROCEDURE ............................................ 4
3.1. Mail ..................................................... 4
3.2. Forwarding ............................................... 7
3.3. Verifying and Expanding .................................. 8
3.4. Sending and Mailing ..................................... 10
3.5. Opening and Closing ..................................... 12
3.6. Relaying ................................................ 13
3.7. Domains ................................................. 15
4. THE SMTP SPECIFICATIONS ...................................... 16
4.1. SMTP Commands ........................................... 16
4.1.1. Command Semantics ..................................... 16
4.1.2. Command Syntax ........................................ 23
4.2. SMTP Replies ............................................ 28
4.2.1. Reply Codes by Function Group ......................... 29
4.2.2. Reply Codes in Numeric Order .......................... 30
4.3. Sequencing of Commands and Replies ...................... 31
4.4. State Diagrams .......................................... 33
4.5. Details ................................................. 35
4.5.1. Minimum Implementation ................................ 35
4.5.2. Transparency .......................................... 35
4.5.3. Sizes ................................................. 36
APPENDIX A: TCP ................................................. 38
APPENDIX B: NCP ................................................. 39
APPENDIX C: NITS ................................................ 40
APPENDIX D: X.25 ................................................ 41
APPENDIX E: Theory of Reply Codes ............................... 42
APPENDIX F: Scenarios ........................................... 45
GLOSSARY ......................................................... 58
REFERENCES ....................................................... 61
Network Working Group J. Postel
Request for Comments: 788 ISI
Replaces: RFC 780, 772 November 1981
SIMPLE MAIL TRANSFER PROTOCOL
1. INTRODUCTION
The objective of Simple Mail Transfer Protocol (SMTP) is to transfer
mail reliably and efficiently.
SMTP is independent of the particular transmission subsystem and
requires only a reliable ordered data stream channel. Appendices A,
B, C, and D describe the use of SMTP with various transport services.
A Glossary provides the definitions of terms as used in this
document.
An important feature of SMTP is its capability to relay mail across
transport service environments. A transport service provides an
interprocess communication environment (IPCE). An IPCE may cover one
network, several networks, or a subset of a network. It is important
to realize that transport systems (or IPCEs) are not one-to-one with
networks. A process can communicate directly with another process
through any mutually known IPCE. Mail is an application or use of
interprocess communication. Mail can be communicated between
processes in different IPCEs by relaying through a process connected
to two (or more) IPCEs. More specifically, mail can be relayed
between hosts on different transport systems by a host on both
transport systems.
November 1981 RFC 788
Simple Mail Transfer Protocol
2. THE SMTP MODEL
The SMTP design is based on the following model of communication: as
the result of a user mail request, the sender-SMTP establishes a
full-duplex transmission channel to a receiver-SMTP. The
receiver-SMTP may be either the ultimate destination or an
intermediate. SMTP commands are generated by the sender-SMTP and
sent to the receiver-SMTP. SMTP replies are sent from the
receiver-SMTP to the sender-SMTP in response to the commands.
Once the transmission channel is established, the SMTP-sender sends a
MAIL command indicating the sender of the mail. If the SMTP-receiver
can accept mail it responds with an OK reply. The SMTP-sender then
sends a RCPT command identifying a recipient of the mail. If the
SMTP-receiver can accept mail for that recipient it responds with an
OK reply; if not, it responds with a reply rejecting that recipient
(but not the whole mail transaction). The SMTP-sender and
SMTP-receiver may negotiate several recipients. When the recipients
have been negotiated the SMTP-sender sends the mail data, terminating
with a special sequence. If the SMTP-receiver successfully processes
the mail data it responds with an OK reply. The dialog is purposely
lock-step, one-at-a-time.
-------------------------------------------------------------
+----------+ +----------+
+------+ | | | |
| User |<-->| | SMTP | |
+------+ | Sender- |Commands/Replies| Receiver-|
+------+ | SMTP |<-------------->| SMTP | +------+
| File |<-->| | and Mail | |<-->| File |
|System| | | | | |System|
+------+ +----------+ +----------+ +------+
Sender-SMTP Receiver-SMTP
Model for SMTP Use
Figure 1
-------------------------------------------------------------
The SMTP provides mechanisms for the transmission of mail; directly
from the sending user's host to the receiving user's host when the
RFC 788 November 1981
Simple Mail Transfer Protocol
two host are connected to the same transport service, or via one or
more relay SMTP-servers when the source and destination hosts are not
connected to the same transport service.
To be able to provide the relay capability the SMTP-server must be
supplied with the name of the ultimate destination host as well as
the destination mailbox name.
The argument to the MAIL command is a reverse-path, which specifies
who the mail is from. The argument to the RCPT command is a
forward-path, which specifies who the mail is to. The forward-path
is a source route while the reverse-path, is a return route (which
may be used to return a message to the sender when an error occurs
with a relayed message).
When the same message is sent to multiple recipients the SMTP
encourages the transmission of only one copy of the data for all the
recipients at the same destination host.
The mail commands and replies have a rigid syntax. Replies also have
a numeric code. In the following, examples appear which use actual
commands and replies. The complete lists of commands and replies
appears in Section 4 on specifications.
Commands and replies are not case sensitive. That is, a command or
reply word may be upper case, lower case, or any mixture of upper and
lower case. Note that this is not true of mailbox user names. For
some hosts the user name is case sensitive, and SMTP implementations
must take case to preserve the case of user names as they appear in
mailbox arguments. Host names are not case sensitive.
Commands and replies are composed of characters from the ASCII
character set [1]. Each 7-bit character is transmitted right
justified in an 8-bit byte (or octet) with the high order bit cleared
to zero.
When specifying the general form of a command or reply, an argument
(or special symbol) will be denoted by a meta-linguistic variable (or
constant), for example, "<string>" or "<reverse-path>". Here the
angle brackets indicate these are a meta-linguistic variables.
However, some arguments use the angle brackets literally. For
example, an actual reverse-path is enclosed in angle brackets, i.e.,
"<Smith@ISIA>" is an instance of <reverse-path> (the angle brackets
are actually transmitted in the command or reply).
November 1981 RFC 788
Simple Mail Transfer Protocol
3. THE SMTP PROCEDURES
This section presents the procedures used in SMTP in several parts.
First comes the basic mail procedure defined as a mail transaction.
Following this are descriptions of forwarding mail, verifying mailbox
names and expanding mailing lists, sending to terminals instead of or
in combination with mailboxes, and the opening and closing exchanges.
At the end of this section are comments on relaying, and a note on
mail domains. Throughout this section are examples of partial
command and reply sequences, several complete scenarios are presented
in Appendix F.
3.1. MAIL
There are three steps to a SMTP mail transaction. The transaction
is started with a MAIL command which gives the sender
identification. A series of one or more RCPT commands follow
giving the receiver information. Then a DATA command gives the
mail data. And finally, the end of mail data indicator confirms
the transaction.
The first step in the procedure is the MAIL command. The
<reverse-path> contains the source mailbox.
MAIL <SP> FROM:<reverse-path> <CRLF>
This command tells the the SMTP-receiver that a new mail
transaction is starting and to reset all its state tables and
buffers including any recipients or mail data. It gives the
reverse-path which can be used to report errors. If accepted,
the receiver-SMTP returns a 250 OK reply.
The <reverse-path> can contain more than just a mailbox. The
<reverse-path> is a reverse source routing list of hosts and
source mailbox. The first host in the <reverse-path> should be
the host sending this command.
The second step in the procedure is the RCPT command.
RCPT <SP> TO:<forward-path> <CRLF>
This command gives a forward-path identifying one recipient.
If accepted, the receiver-SMTP returns a 250 OK reply, and
stores the forward-path. If the recipient is unknown the
receiver-SMTP returns a 550 Failure reply. This second step of
the procedure can be repeated any number of times.
RFC 788 November 1981
Simple Mail Transfer Protocol
The <forward-path> can contain more than just a mailbox. The
<forward-path> is a source routing list of hosts and
destination mailbox. The first host in the <forward-path>
should be the host receiving this command.
The third step in the procedure is the DATA command.
DATA <CRLF>
If accepted, the receiver-SMTP returns a 354 Intermediate reply
and considers all succeeding lines to be the message text.
When the end of text is received and stored the SMTP-receiver
sends a 250 OK reply.
Since the mail data is sent on the transmission channel the end
of the mail data must be indicated so that the command and
reply dialog can be resumed. SMTP indicates the end of the
mail data by sending a line containing only a period. A
transparency procedure is used to prevent this interfering with
the user's text (see Section 4.5.2).
Please note that the mail data includes the memo header
items such as Date, Subject, To, Cc, From [2].
The end of mail data indicator also confirms the mail
transaction and tells the receiver-SMTP to now process the
stored recipients and mail data. If accepted, the
receiver-SMTP returns a 250 OK reply. The DATA command should
fail only if the mail transaction was incomplete (for example,
no recipients), or if resources are not available.
The above procedure is an example of a SMTP mail transaction.
These commands must be used only in the order discussed above.
Example 1 (below) illustrates the use of these commands in a mail
transaction.
November 1981 RFC 788
Simple Mail Transfer Protocol
-------------------------------------------------------------
Example of the SMTP Procedure
This SMTP example shows mail sent by Smith at host Alpha, to
Jones, Green, and Brown at host Beta. Here we assume that host
Alpha contacts host Beta directly.
S: MAIL FROM:<Smith@Alpha>
R: 250 OK
S: RCPT TO:<Jones@Beta>
R: 250 OK
S: RCPT TO:<Green@Beta>
R: 550 No such user here
S: RCPT TO:<Brown@Beta>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: <CRLF>.<CRLF>
R: 250 OK
The mail has now been accepted for Jones and Brown. Green did
not have a mailbox at host Beta.
Example 1
-------------------------------------------------------------
RFC 788 November 1981
Simple Mail Transfer Protocol
3.2. FORWARDING
There are some cases where the destination information in the
<forward-path> is incorrect, but the receiver-SMTP knows the
correct destination. In such cases, one the following replies
should be used to allow the sender to contact the correct
destination.
251 User not local; will forward to <forward-path>
This reply indicates that the receiver-SMTP knows the user's
mailbox is on another host and indicates the correct
forward-path to use in the future. Note that either the
host or user or both may be different. The receiver takes
responsibility for delivering the message.
551 User not local; please try <forward-path>
This reply indicates that the receiver-SMTP knows the user's
mailbox is on another host and indicates the correct
forward-path to use. Note that either the host or user or
both may be different. The receiver refuses to accept mail
for this user, and the sender must either redirect the mail
according to the information provided or return an error
response to the originating user.
Example 2 illustrates the use of these responses.
-------------------------------------------------------------
Example of Forwarding
Either
S: RCPT TO:<Postel@ISI>
R: 251 User not local; will forward to <Postel@ISIF>
Or
S: RCPT TO:<Paul@ISIB>
R: 551 User not local; please try <Mockapetris@ISIF>
Example 2
-------------------------------------------------------------
November 1981 RFC 788
Simple Mail Transfer Protocol
3.3. VERIFYING AND EXPANDING
SMTP provides as additional features, commands to verify a user
name or expand a mailing list. This is done with the VRFY and
EXPN commands, which have a character string arguments. For the
VRFY command, the string is a user name, and the the response may
include the full name of the user and must include the mailbox of
the user. For the EXPN command, the string identifies a mailing
list, and the multiline response may include the full name of the
users and must give the mailboxes on the mailing list.
The case of verifying a user name is straightforward as shown in
example 3.
-------------------------------------------------------------
Example of Verifying a User Name
Either
S: VRFY Postel
R: 250 Jon Postel <Postel@ISIF>
Or
S: VRFY Jones
R: 550 String does not match anything.
Example 3
-------------------------------------------------------------
RFC 788 November 1981
Simple Mail Transfer Protocol
The case of expanding a mailbox list requires a multiline reply as
shown in example 4.
-------------------------------------------------------------
Example of Expanding a Mailing List
Either
S: EXPN Example-People
R: 250-Jon Postel <Postel@ISIF>
R: 250-Fred Fonebone <Fonebone@ISIQ>
R: 250-Sam Q. Smith <SQSmith@ISIQ>
R: 250-Quincy Smith <@ISIF,Q-Smith@ISI-VAXA>
R: 250-<joe@foo-unix>
R: 250 <xyz@bar-unix>
Or
S: EXPN Executive-Washroom-List
R: 550 Access Denied to You.
Example 4
-------------------------------------------------------------
The character string arguments of the VRFY and EXPN commands
cannot be further restricted due to the variety of implementations
of the user name and mailbox list concepts. On some systems it
may be appropriate for the argument of the EXPN command to be a
file name for a file containing a mailing list, but again there is
a variety of file naming conventions in the internet.
The VRFY and EXPN commands are not included in the minimum
implementation (Section 4.5.1), and are not required to work
across relays when they are implemented.
November 1981 RFC 788
Simple Mail Transfer Protocol
3.4. SENDING AND MAILING
The main purpose of SMTP is to deliver messages to user's
mailboxes. A very similar service provided by some hosts is to
deliver messages to user's terminals (provided the user is active
on the host). The delivery to the user's mailbox is called
"mailing", the delivery to the user's terminal is called
"sending". Because in many hosts the implementation of sending is
nearly identical to the implementation of mailing these two
functions are combined in SMTP. However the sending commands are
not included in the required minimum implementation
(Section 4.5.1). User's should have the ability to control the
writing of messages on their terminals. Most hosts permit the
user's to accept or refuse such messages.
The following three command are defined to support the sending
options, these are used in the mail transaction instead of the
MAIL command and inform the receiver-SMTP of the special semantics
of this transaction:
SEND <SP> FROM:<reverse-path> <CRLF>
The SEND command requires that the mail data be delivered to
the user's terminal. If the user is not active (or not
accepting terminal messages) on the host a 450 reply may
returned to a RCPT command. The mail transaction is
successful if the message is delivered the terminal.
SOML <SP> FROM:<reverse-path> <CRLF>
The Send Or MaiL command requires that the mail data be
delivered to the user's terminal if the user is active (and
accepting terminal messages) on the host. If the user is
not active (or not accepting terminal messages) then the
mail data is entered into the user's mailbox. The mail
transaction is successful if the message is delivered either
to the terminal or the mailbox.
SAML <SP> FROM:<reverse-path> <CRLF>
The Send And MaiL command requires that the mail data be
delivered to the user's terminal if the user is active (and
accepting terminal messages) on the host. In any case the
mail data is entered into the user's mailbox. The mail
transaction is successful if the message is delivered the
mailbox.
RFC 788 November 1981
Simple Mail Transfer Protocol
The same reply codes that are used for the MAIL commands are used
for these commands.
November 1981 RFC 788
Simple Mail Transfer Protocol
3.5. OPENING AND CLOSING
At the time the transmission channel is opened there is an
exchange to ensure that the hosts are communicating with the hosts
they think they are.
The following two commands are used in transmission channel
opening and closing:
HELO <SP> <host> <CRLF>
QUIT <CRLF>
In the HELO command the host sending the command identifies
itself; the command may be interpreted as saying "Hello, i am
<host>".
-------------------------------------------------------------
Example of Connection Opening
R: 220 BBN-UNIX Simple Mail Transfer Service Ready
S: HELO USC-ISIF
R: 250 BBN-UNIX
Example 5
-------------------------------------------------------------
-------------------------------------------------------------
Example of Connection Closing
S: QUIT
R: 221 BBN-UNIX Service closing transmission channel
Example 6
-------------------------------------------------------------
RFC 788 November 1981
Simple Mail Transfer Protocol
3.6. RELAYING
The forward-path may be a source route of the form
"@ONE,@TWO,JOE@THREE", where ONE, TWO, and THREE are hosts. This
form is used to emphasize the distinction between an address and a
route. The mailbox is an absolute address, and the route is
information about how to get there. The two concepts should not
be confused.
The elements of the forward-path are moved to the reverse-path as
the message is relayed from one server-SMTP to another. The
reverse-path is a reverse source route, (i.e., a source route from
the current location of the message to the originator of the
message). When a server-SMTP deletes its identifier from the
forward-path and inserts it into the reverse-path, it must use the
name it is known by in the environment it is sending into, not the
environment the mail came from, in case the server-SMTP is known
by different names in different environments.
Using source routing the receiver-SMTP receives mail to be relayed
to another server-SMTP The receiver-SMTP may accept or reject the
task of relaying the mail in the same way it accepts or rejects
mail for a local user. The receiver-SMTP transforms the command
arguments by moving its own identifier from the forward-path to
the beginning of the reverse-path. The receiver-SMTP then becomes
a sender-SMTP, establishes a transmission channel to the next SMTP
in the forward-path, and sends it the mail.
The first host in the reverse-path should be the host sending the
SMTP commands, and the first host in the forward-path should be
the host receiving the SMTP commands.
Notice that the forward-path and reverse-path appear in the SMTP
commands and replies, but not necessarily in the message. That
is, there is no need for these paths and especially this syntax to
appear in the "To:" , "From:", "CC:", etc. fields of the message
header.
If a server-SMTP has accepted the task of relaying the mail and
later finds that the forward-path is incorrect or that the mail
cannot be delivered for whatever reason, then it must construct an
"undeliverable mail" notification message and send it to the
originator of the undeliverable mail (as indicated by the
reverse-path).
November 1981 RFC 788
Simple Mail Transfer Protocol
This notification message must be from the server-SMTP at this
host. Of course, server-SMTPs should not send notification
messages about problems with notification messages. One way to
prevent loops in error reporting is to specify a null reverse-path
in the MAIL command of a notification message. When such a
message is relayed it is permissible to leave the reverse-path
null. A MAIL command with a null reverse-path appears as follows:
MAIL FROM:<>
An undeliverable mail notification message is shown in example 7.
This notification is in response to a message originated by JOE at
HOSTW and sent via HOSTX to HOSTY with instructions to relay it on
to HOSTZ. What we see in the example is the transaction between
HOSTY and HOSTX, which is the first step in the return of the
notification message.
-------------------------------------------------------------
Example Undeliverable Mail Notification Message
S: MAIL FROM:<>
R: 250 ok
S: RCPT TO:<@HOSTX,JOE@HOSTW>
R: 250 ok
S: DATA
R: 354 send the mail data, end with .
S: Date: 23 Oct 81
S: Sender: SMTP@HOSTY
S: Subject: Mail System Problem
S:
S: Sorry JOE, your message to SAM@HOSTZ lost.
S: HOSTZ said this:
S: "550 No Such User"
S: .
R: 250 ok
Example 7
-------------------------------------------------------------
RFC 788 November 1981
Simple Mail Transfer Protocol
3.7. DOMAINS
At some not too distant future time it might be necessary to
expand the mailbox format to include a region or name domain
identifier. There is quite a bit of discussion on this at
present, and is likely that SMTP will be revised in the future to
take into account naming domains.
The examples in this document do not show mail domains.
November 1981 RFC 788
Simple Mail Transfer Protocol
4. THE SMTP SPECIFICATIONS
4.1. SMTP COMMANDS
4.1.1. COMMAND SEMANTICS
The SMTP commands define the mail transfer or the mail system
function requested by the user. SMTP commands are character
strings terminated by <CRLF>. The command codes themselves are
alphabetic characters terminated by <SP> if parameters follow
and <CRLF> otherwise. The syntax of mailboxes must conform to
receiver site conventions. The SMTP commands are discussed
below. The SMTP replies are discussed in the Section 4.2.
A mail transaction involves several data objects which are
communicated as arguments to different commands. The
reverse-path is the argument of the MAIL command, the
forward-path is the argument of the RCPT command, and the mail
data is the argument of the DATA command. These arguments or
data objects must be transmitted and held pending the
confirmation communicated by the end of mail data indication
which finalizes the transaction. The model for this is that
distinct buffers are provided to hold the types of data
objects, that is, there is a reverse-path buffer, a
forward-path buffer, and a mail data buffer. Specific commands
cause information to be appended to a specific buffer, or cause
one or more buffers to be cleared.
HELLO (HELO)
This command is used to identify the sender-SMTP to the
receiver-SMTP. The argument field contains the host name of
the sender-SMTP.
The receiver-SMTP identifies itself to the sender-SMTP in
the connection greeting reply, and in the response to this
command.
MAIL (MAIL)
This command is used to initiate a mail transaction in which
the mail data is delivered to one or more mailboxes. The
argument field contains a reverse-path.
The reverse-path consists of an optional list of hosts and
the sender mailbox. When the list of hosts is present, it
RFC 788 November 1981
Simple Mail Transfer Protocol
is a "reverse" source route and indicates that the mail was
relayed through each host on the list (the first host in the
list was the most recent relay). This list is used as a
source route to return non-delivery notices to the sender.
As each relay host adds itself to the beginning of the list,
it must use its name as known in the IPCE to which it is
relaying the mail rather than the IPCE from which the mail
came (if they are different). In some types of error
reporting messages (for example, undeliverable mail
notifications) the reverse-path may be null (see Example 7).
This command clears the reverse-path buffer, the
forward-path buffer, and the mail data buffer; and inserts
the reverse-path information from this command into the
reverse-path buffer.
RECIPIENT (RCPT)
This command is used to identify an individual recipient of
the mail data; multiple recipients are specified by multiple
use of this command.
The forward-path consists of an optional list of hosts and a
required destination mailbox. When the list of hosts is
present, it is a source route and indicates that the mail
must be relayed to the next host on the list. If the
receiver-SMTP is does not implement the relay function it
may user the same reply it would for an unknown local user
(550).
When mail is relayed, the relay host must remove itself from
the beginning forward-path and put itself at the beginning
of the reverse-path. When mail reaches its ultimate
destination (the forward-path contains only a destination
mailbox), the receiver-SMTP inserts it into the destination
mailbox in accordance with its host mail conventions.
For example, mail received at relay host A with arguments
FROM:<X@Y>
TO:<@A,@B,C@D>
will be relayed on to host B with arguments
FROM:<@A,X@Y>
TO:<@B,C@D>.
November 1981 RFC 788
Simple Mail Transfer Protocol
This command causes its forward-path argument to be appended
to the forward-path buffer.
DATA (DATA)
The receiver treats the lines following the command as mail
data from the sender. This command causes the mail data
from this command to be appended to the mail data buffer.
The mail data may contain any of the 128 ASCII character
codes.
The mail data is terminated by a line containing only a
period, that is the character sequence "<CRLF>.<CRLF>" (see
Section 4.5.2 on Transparency). This is the end of mail
data indication.
The end of mail data indication requires that the receiver
must now process the stored mail transaction information.
This processing consumes the information in the reverse-path
buffer, the forward-path buffer, and the mail data buffer,
and on the completion of this command these buffers are
cleared. If the processing is successful the receiver must
send an OK reply. If the processing fails completely the
receiver must send a failure reply.
When the receiver-SMTP accepts a message either for relaying
or for final delivery it inserts at the beginning of the
mail data a time stamp line. The time stamp line indicates
the identity of the host that sent the message, and the
identity of the host that received the message (and is
inserting this time stamp), and the date and time the
message was received. Relayed messages will have multiple
time stamp lines.
When the receiver-SMTP makes the "final delivery" of a
message it inserts at the beginning of the mail data a
return path line. The return path line preserves the
information in the <reverse-path> from the MAIL command.
Here, final delivery means the message leaves the SMTP
world. Normally, this would mean it has been delivered to
the destination user, but in some cases it may be further
processed and transmitted by another mail system.
The preceding two paragraphs imply that the final mail data
RFC 788 November 1981
Simple Mail Transfer Protocol
will begin with a return path line, followed by one or more
time stamp lines. These lines will be followed by the mail
data header and body [2]. For example:
Return-Path: <@GHI,@DEF,@ABC,JOE@ABC>
Mail-From: GHI received by JKL at 27-Oct-81 15:27:39-PST
Mail-From: DEF received by GHI at 27-Oct-81 15:15:13-PST
Mail-From: ABC received by DEF at 27-Oct-81 15:01:59-PST
Date: 27-Oct-81 15:01:01-PST
From: JOE@ABC
Subject: Improved Mailing System Installed
To: SAM@JKL
This is to inform you that ...
Special mention is needed of the response and further action
required when the processing following the end of mail data
indication is partially successful. This could arise if
after accepting several recipients and the mail data, the
receiver-SMTP finds that the mail data can be successfully
delivered to some of the recipients, but it cannot be to
others (for example, due to mailbox space allocation
problems). In such a situation, the response to the DATA
command must be an OK reply. But, the receiver-SMTP must
compose and send an "undeliverable mail" notification
message to the originator of the message. Either a single
notification which lists all of the recipients that failed
to get the message, or separate notification messages must
be sent for each failed recipient (see Example 7). All
undeliverable mail notification messages are sent using the
MAIL command (even if they result from processing a SEND,
SOML, or SAML command).
SEND (SEND)
This command is used to initiate a mail transaction in which
the mail data is delivered to one or more terminals. The
argument field contains a reverse-path. This command is
successful if the message is delivered to the terminal.
The reverse-path consists of an optional list of hosts and
the sender mailbox. When the list of hosts is present, it
is a "reverse" source route and indicates that the mail was
relayed through each host on the list (the first host in the
list was the most recent relay). This list is used as a
source route to return non-delivery notices to the sender.
November 1981 RFC 788
Simple Mail Transfer Protocol
As each relay host adds itself to the beginning of the list,
it must use its name as known in the IPCE to which it is
relaying the mail rather than the IPCE from which the mail
came (if they are different).
This command clears the reverse-path buffer, the
forward-path buffer, and the mail data buffer; and inserts
the reverse-path information from this command into the
reverse-path buffer.
SEND OR MAIL (SOML)
This command is used to initiate a mail transaction in which
the mail data is delivered to one or more terminals or
mailboxes. For each recipient the mail data is delivered to
the recipient's terminal if the recipient is active on the
host (and accepting terminal messages), otherwise to the
recipient's mailbox. The argument field contains a
reverse-path. This command is successful if the message is
delivered to the terminal or the mailbox.
The reverse-path consists of an optional list of hosts and
the sender mailbox. When the list of hosts is present, it
is a "reverse" source route and indicates that the mail was
relayed through each host on the list (the first host in the
list was the most recent relay). This list is used as a
source route to return non-delivery notices to the sender.
As each relay host adds itself to the beginning of the list,
it must use its name as known in the IPCE to which it is
relaying the mail rather than the IPCE from which the mail
came (if they are different).
This command clears the reverse-path buffer, the
forward-path buffer, and the mail data buffer; and inserts
the reverse-path information from this command into the
reverse-path buffer.
SEND AND MAIL (SAML)
This command is used to initiate a mail transaction in which
the mail data is delivered to one or more terminals and
mailboxes. For each recipient the mail data is delivered to
the recipient's terminal if the recipient is active on the
host (and accepting terminal messages), and for all
RFC 788 November 1981
Simple Mail Transfer Protocol
recipients to the recipient's mailbox. The argument field
contains a reverse-path. This command is successful if the
message is delivered to the mailbox.
The reverse-path consists of an optional list of hosts and
the sender mailbox. When the list of hosts is present, it
is a "reverse" source route and indicates that the mail was
relayed through each host on the list (the first host in the
list was the most recent relay). This list is used as a
source route to return non-delivery notices to the sender.
As each relay host adds itself to the beginning of the list,
it must use its name as known in the IPCE to which it is
relaying the mail rather than the IPCE from which the mail
came (if they are different).
This command clears the reverse-path buffer, the
forward-path buffer, and the mail data buffer; and inserts
the reverse-path information from this command into the
reverse-path buffer.
RESET (RSET)
This command specifies that the current mail transaction is
to be aborted. Any stored sender, recipients, and mail data
must be discarded, and all buffers and state tables cleared.
The receiver must send an OK reply.
VERIFY (VRFY)
This command asks the receiver to confirm that the argument
identifies a user. If it is a user name, the full name of
the user (if known) and the fully specified mailbox are
returned.
This command has no effect on any of the reverse-path
buffer, the forward-path buffer, or the mail data buffer.
EXPAND (EXPN)
This command asks the receiver to confirm that the argument
identifies a mailing list, and if so, to return the
membership of that list. The full name of the users (if
known) and the fully specified mailboxes are returned in a
multiline reply.
November 1981 RFC 788
Simple Mail Transfer Protocol
This command has no effect on any of the reverse-path
buffer, the forward-path buffer, or the mail data buffer.
HELP (HELP)
This command causes the receiver to send helpful information
to the sender of the HELP command. The command may take an
argument (e.g., any command name) and return more specific
information as a response.
This command has no effect on any of the reverse-path
buffer, the forward-path buffer, or the mail data buffer.
NOOP (NOOP)
This command does not affect any parameters or previously
entered commands. It specifies no action other than that
the receiver send an OK reply.
This command has no effect on any of the reverse-path
buffer, the forward-path buffer, or the mail data buffer.
QUIT (QUIT)
This command specifies that the receiver must send an OK
reply, and then close the transmission channel.
The receiver should not close the transmission channel until
it receives and replies to a QUIT command (even if there was
an error). The sender should not close the transmission
channel until it send a QUIT command and receives the reply
(even if there was an error response to a previous command).
If the connection is closed prematurely the receiver should
act as if a RSET command had been received (canceling any
pending transaction, but not undoing any previously
completed transaction), the sender should act as if the
command or transaction in progress had received a temporary
error (4xx).
There are restrictions on the order in which these command may
be used.
The first command in a session must be the HELO command.
The HELO command may be used later in a session as well.
RFC 788 November 1981
Simple Mail Transfer Protocol
The NOOP, HELP, EXPN, and VRFY commands can be used at any
time during a session.
The MAIL, SEND, SOML, or SAML commands begin a mail
transaction. Once started a mail transaction consists of
one of the transaction beginning commands, one or more RCPT
commands, and a DATA command, in that order. A mail
transaction may be aborted by the RSET command. There may
be zero or more transactions in a session.
The last command in a session must be the QUIT command. The
QUIT command can not be used at any other time in a session.
4.1.2. COMMAND SYNTAX
The commands consist of a command code followed by an argument
field. Command codes are four alphabetic characters. Upper
and lower case alphabetic characters are to be treated
identically. Thus, any of the following may represent the mail
command:
MAIL Mail mail MaIl mAIl
This also applies to any symbols representing parameter values,
such as "TO" or "to" for the forward-path. Command codes and
the argument fields are separated by one or more spaces.
However, within the reverse-path and forward-path arguments
case is important. In particular, in some hosts the user
"smith" is different from the user "Smith".
The argument field consists of a variable length character
string ending with the character sequence <CRLF>. The receiver
is to take no action until this sequence is received.
Square brackets denote an optional argument field. If the
option is not taken, the appropriate default is implied.
November 1981 RFC 788
Simple Mail Transfer Protocol
The following are the SMTP commands:
HELO <SP> <host> <CRLF>
MAIL <SP> FROM:<reverse-path> <CRLF>
RCPT <SP> TO:<forward-path> <CRLF>
DATA <CRLF>
RSET <CRLF>
SEND <SP> FROM:<reverse-path> <CRLF>
SOML <SP> FROM:<reverse-path> <CRLF>
SAML <SP> FROM:<reverse-path> <CRLF>
VRFY <SP> <string> <CRLF>
EXPN <SP> <string> <CRLF>
HELP [<SP> <string>] <CRLF>
NOOP <CRLF>
QUIT <CRLF>
RFC 788 November 1981
Simple Mail Transfer Protocol
The syntax of the above argument fields (using BNF notation
where applicable) is given below. The "..." notation indicates
that a field may be repeated one or more times.
<reverse-path> ::= <path>
<forward-path> ::= <path>
<path> ::= "<" ["@" <host> "," ...] <mailbox> ">"
<host> ::= <a> <string> | "#" <number> | "[" <dotnum> "]"
<mailbox> ::= <user> "@" <host>
<user> ::= <string>
<string> ::= <char> | <char> <string>
<char> ::= <c> | '\' <c> | '\' <s>
<dotnum> ::= <snum> "." <snum> "." <snum> "." <snum>
<number> ::= <d> | <d> <number>
<snum> ::= three digits representing a decimal integer value
in the range 0 through 255
<a> ::= any one of the 52 alphabetic characters A through Z
in upper case and a through z in lower case
<c> ::= any one of the 128 ASCII characters except
<specials>
<d> ::= any one of the ten digits 0 through 9
<s> ::= any one of <specials>
<specials> ::= '<', '>', '(', ')', '\', ',', ';', ':', '@',
'"', and the control characters (ASCII codes 0 through 37
octal inclusive and 177 octal)
Note that the backslash, '\', is a quote character, which is
used to indicate that the next character is to be used
literally (instead of its normal interpretation). For example,
"Joe\,Smith" could be used to indicate a single nine character
user field with comma being the fourth character of the field.
November 1981 RFC 788
Simple Mail Transfer Protocol
Hosts are generally known by names which are translated to
addresses in each host. Sometimes a host is not known to the
translation function and communication is blocked. To bypass
this barrier two numeric forms are also allowed for host
"names". One form is a decimal integer prefixed by a pound
sign, "#", which indicates the number is the address of the
host. Another form is four small decimal integers separated by
dots and enclosed by brackets, e.g., "[123.255.37.2]", which
indicates a 32-bit ARPA Internet Address in four 8-bit fields.
The time stamp line and the return path line are formally
defined as follows:
<return-path-line> ::= "Return-Path:" <SP><reverse-path><CRLF>
<time-stamp-line> ::= "Mail-From:" <SP> <stamp> <CRLF>
<stamp> ::= [<ptcl>] <from-host> <this-host> <daytime>
<ptcl> ::= <protocol> <SP> "host" <SP>
<from-host> ::= <host> <SP>
<this-host> ::= "received by" <SP> <host> <SP>
<protocol> ::= "TCP" | "NCP" | "NITS" | "X25" | "INTERNET" |
"ARPANET"
Note: INTERNET = TCP, ARPANET = NCP, and if the <ptcl> is
not present INTERNET is assumed.
<daytime> ::= "at" <SP> <date> <SP> <time>
<date> ::= <dd> "-" <mon> "-" <yy>
<time> ::= <hh> ":" <mm> ":" <ss> "-" <zone>
<dd> ::= the one or two decimal integer day of the month in
the range 1 to 31.
<mon> ::= "JAN" | "FEB" | "MAR" | "APR" | "MAY" | "JUN" |
"JUL" | "AUG" | "SEP" | "OCT" | "NOV" | "DEC"
<yy> ::= the two decimal integer year of the century in the
range 01 to 99.
RFC 788 November 1981
Simple Mail Transfer Protocol
<hh> ::= the two decimal integer hour of the day in the
range 00 to 24.
<mm> ::= the two decimal integer minute of the hour in the
range 00 to 59.
<ss> ::= the two decimal integer second of the minute in the
range 00 to 59.
<zone> ::= a time zone designator (as in [2]) or "UT" for
Universal Time (the default).
Return Path Example:
Return-Path: <@CHARLIE,@BAKER,JOE@ABLE>
Mail From Example:
Mail-From: ABC received by XYZ at 22-OCT-81 09:23:59-PDT
November 1981 RFC 788
Simple Mail Transfer Protocol
4.2. SMTP REPLIES
Replies to SMTP commands are devised to ensure the synchronization
of requests and actions in the process of mail transfer, and to
guarantee that the sender-SMTP always knows the state of the
receiver-SMTP. Every command must generate exactly one reply.
The details of the command-reply sequence are made explicit in
Section 5.3 on Sequencing and Section 5.4 State Diagrams.
An SMTP reply consists of a three digit number (transmitted as
three alphanumeric characters) followed by some text. The number
is intended for use by automata to determine what state to enter
next; the text is meant for the human user. It is intended that
the three digits contain enough encoded information that the
sender-SMTP need not examine the text and may either discard it or
pass it on to the user, as appropriate. In particular, the text
may be receiver-dependent, so there are likely to be varying texts
for each reply code. A discussion of the theory of reply codes is
given in the Appendix E. Formally, a reply is defined to be the
sequence: a three-digit code, <SP>, one line of text, and <CRLF>,
or a multiline reply (as defined in Appendix E). Only the EXPN
and HELP command are expected to result in multiline replies in
normal circumstances, however multiline replies are allowed for
any command.
RFC 788 November 1981
Simple Mail Transfer Protocol
4.2.1. REPLY CODES BY FUNCTION GROUPS
500 Syntax error, command unrecognized
[This may include errors such as command line too long]
501 Syntax error in parameters or arguments
502 Command not implemented
503 Bad sequence of commands
504 Command parameter not implemented
211 System status, or system help reply
214 Help message
[Information on how to use the receiver or the meaning of a
particular non-standard command; this reply is useful only
to the human user]
220 <host> Service ready
221 <host> Service closing transmission channel
421 <host> Service not available, closing transmission channel
[This may be a reply to any command if the service knows it
must shut down]
250 Requested mail action okay, completed
251 User not local; will forward to <forward-path>
450 Requested mail action not taken: mailbox unavailable
[E.g., mailbox busy]
550 Requested action not taken: mailbox unavailable
[E.g., mailbox not found, no access]
451 Requested action aborted: error in processing
551 User not local; please try <forward-path>
452 Requested action not taken: insufficient system storage
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed
[E.g., mailbox syntax incorrect]
354 Start mail input; end with <CRLF>.<CRLF>
554 Transaction failed
November 1981 RFC 788
Simple Mail Transfer Protocol
4.2.2. NUMERIC ORDER LIST OF REPLY CODES
211 System status, or system help reply
214 Help message
[Information on how to use the receiver or the meaning of a
particular non-standard command; this reply is useful only
to the human user]
220 <host> Service ready
221 <host> Service closing transmission channel
250 Requested mail action okay, completed
251 User not local; will forward to <forward-path>
354 Start mail input; end with <CRLF>.<CRLF>
421 <host> Service not available, closing transmission channel
[This may be a reply to any command if the service knows it
must shut down]
450 Requested mail action not taken: mailbox unavailable
[E.g., mailbox busy]
451 Requested action aborted: local error in processing
452 Requested action not taken: insufficient system storage
500 Syntax error, command unrecognized
[This may include errors such as command line too long]
501 Syntax error in parameters or arguments
502 Command not implemented
503 Bad sequence of commands
504 Command parameter not implemented
550 Requested action not taken: mailbox unavailable
[E.g., mailbox not found, no access]
551 User not local; please try <forward-path>
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed
[E.g., mailbox syntax incorrect]
554 Transaction failed
RFC 788 November 1981
Simple Mail Transfer Protocol
4.3. SEQUENCING OF COMMANDS AND REPLIES
The communication between the sender and receiver is intended to
be an alternating dialogue, controlled by the sender. As such,
the sender issues a command and the receiver responds with a
reply. The sender must wait for this response before sending
further commands.
One important reply is the connection greeting. Normally, a
receiver will send a 220 "Awaiting input" reply when the
connection is completed. The sender should wait for this greeting
message before sending any commands.
Note: all the greeting type replies have the official name of
the server host as the first word following the reply code.
For example,
220 <SP> USC-ISIF <SP> Service ready <CRLF>
The table below lists alternative success and failure replies for
each command. These must be strictly adhered to; a receiver may
substitute text in the replies, but the meaning and action implied
by the code numbers and by the specific command reply sequence
cannot be altered.
COMMAND-REPLY SEQUENCES
Each command is listed with its possible replies. The prefixes
used before the possible replies are "P" for preliminary (not
used in SMTP), "I" for intermediate, "S" for success, "F" for
failure, and "E" for error. The 421 reply (service not
available, closing transmission channel) may be given to any
command if the SMTP-receiver knows it must shut down. This
listing forms the basis for the State Diagrams in Section 4.4.
CONNECTION ESTABLISHMENT
S: 220
F: 421
HELO
S: 250
E: 500, 501, 504, 421
MAIL
S: 250
F: 552, 451, 452
E: 500, 501, 421
November 1981 RFC 788
Simple Mail Transfer Protocol
RCPT
S: 250, 251
F: 550, 551, 552, 553, 450, 451, 452
E: 500, 501, 421
DATA
I: 354 -> data -> S: 250
F: 552, 554, 451, 452
F: 451, 554
E: 500, 501, 421
RSET
S: 250
E: 500, 501, 504, 421
SEND
S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
SOML
S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
SAML
S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
VRFY
S: 250
F: 550
E: 500, 501, 502, 504, 421
EXPN
S: 250
F: 550
E: 500, 501, 502, 504, 421
HELP
S: 211, 214
E: 500, 501, 502, 504, 421
NOOP
S: 250
E: 500, 421
QUIT
S: 221
E: 500
RFC 788 November 1981
Simple Mail Transfer Protocol
4.4. STATE DIAGRAMS
Following are state diagrams for a simple-minded SMTP
implementation. Only the first digit of the reply codes is used.
There is one state diagram for each group of SMTP commands. The
command groupings were determined by constructing a model for each
command and then collecting together the commands with
structurally identical models.
For each command there are three possible outcomes: "success"
(S), "failure" (F), and "error" (E). In the state diagrams below
we use the symbol B for "begin", and the symbol W for "wait for
reply".
First, the diagram that represents most of the SMTP commands:
1,3 +---+
----------->| E |
| +---+
|
+---+ cmd +---+ 2 +---+
| B |---------->| W |---------->| S |
+---+ +---+ +---+
|
| 4,5 +---+
----------->| F |
+---+
This diagram models the commands:
HELO, MAIL, RCPT, RSET, SEND, SOML, SAML, VRFY, EXPN, HELP,
NOOP, QUIT.
November 1981 RFC 788
Simple Mail Transfer Protocol
A more complex diagram models the DATA command:
+---+ DATA +---+ 1,2 +---+
| B |---------->| W |-------------------->| E |
+---+ +---+ ------------>+---+
3| |4,5 |
| | |
-------------- ----- |
| | | +---+
| ---------- -------->| S |
| | | | +---+
| | ------------
| | | |
V 1,3| |2 |
+---+ data +---+ --------------->+---+
| |---------->| W | | F |
+---+ +---+-------------------->+---+
4,5
Note that the "data" here is a series of lines sent from the
sender to the receiver with no response expected until the last
line is sent.
RFC 788 November 1981
Simple Mail Transfer Protocol
4.5. DETAILS
4.5.1. MINIMUM IMPLEMENTATION
In order to make SMTP workable, the following minimum
implementation is required for all receivers:
COMMANDS -- HELO
MAIL
RCPT
DATA
RSET
NOOP
QUIT
4.5.2. TRANSPARENCY
Without some provision for data transparency the character
sequence "<CRLF>.<CRLF>" ends the the mail text and cannot be
sent by the user. In general, users are not aware of such
"forbidden" sequences. To allow all user composed text to be
transmitted transparently the following procedures are used.
1. Before sending a line of mail text the sender-SMTP checks
the first character of the line. If it is a period, one
additional period is inserted at the beginning of the line.
2. When a line of mail text is received by the receiver-SMTP
it checks the the line. If the line is composed of a single
period it is the end of mail. If the first character is a
period and there are other characters on the line, the first
character is deleted.
The mail data may contain any of the 128 ASCII characters. All
characters are to be delivered to the recipients mailbox
including format effectors and other control characters. The
7-bit ASCII codes are transmitted right justified in 8-bit
bytes (octets) with the high order bits cleared to zero.
In some systems it may be necessary to transform the data as
it is received and stored. This may be necessary for hosts
that use a different character set than ASCII as their local
character set, or that store data in records rather than
strings. If such transforms are necessary, they must be
reversible -- especially if such transforms are applied to
mail being relayed.
November 1981 RFC 788
Simple Mail Transfer Protocol
4.5.3. SIZES
There are several objects that have required minimum maximum
sizes. That is every implementation must be able to receive
objects of at least these sizes, but must not send objects
larger than these sizes.
****************************************************
* *
* TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION *
* TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH *
* OF THESE OBJECTS SHOULD BE USED. *
* *
****************************************************
user
The maximum total length of a user name is 64 characters.
host
The maximum total length of a host name or number is 40
characters.
path
The maximum total length of a reverse-path or
forward-path is 256 characters (including the punctuation
and element separators).
command line
The maximum total length of a command line including the
command word and the <CRLF> is 512 characters.
reply line
The maximum total length of a reply line including the
reply code and the <CRLF> is 512 characters.
text line
The maximum total length of a text line including the
<CRLF> is 1000 characters (but not counting the leading
dot duplicated for transparency).
RFC 788 November 1981
Simple Mail Transfer Protocol
recipients buffer
The maximum total number of recipients that must be
buffered is 100 recipients.
****************************************************
* *
* TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION *
* TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH *
* OF THESE OBJECTS SHOULD BE USED. *
* *
****************************************************
Errors due to exceeding these limits may be reported by using
the reply codes, for example:
500 Line too long.
501 Path too long
552 Too many recipients.
552 Too much mail data.
November 1981 RFC 788
Simple Mail Transfer Protocol
APPENDIX A
TCP Transport service
The Transmission Control Protocol [3] is used in the ARPA
Internet, and in any network following the US DoD standards for
internetwork protocols.
Connection Establishment
The SMTP transmission channel is a TCP connection established
between the sender process port U and the receiver process port
L. This single full duplex connection is used as the
transmission channel. This protocol is assigned the service
port 25 (31 octal), that is L=25.
Data Transfer
The TCP connection supports the transmission of 8-bit bytes.
The SMTP data is 7-bit ASCII characters. Each character is
transmitted as a 8-bit byte with the high-order bit cleared to
zero.
RFC 788 November 1981
Simple Mail Transfer Protocol
APPENDIX B
NCP Transport service
The ARPANET Host-to-Host Protocol [4] (implemented by the Network
Control Program) may be used in the ARPANET.
Connection Establishment
The SMTP transmission channel is established via NCP between
the the sender process socket U and receiver process socket L.
The Initial Connection Protocol [5] is followed resulting in a
pair of simplex connections. This pair of connections is used
as the transmission channel. This protocol is assigned the
contact socket 25 (31 octal), that is L=25.
Data Transfer
The NCP data connections are established in 8-bit byte mode.
The SMTP data is 7-bit ASCII characters. Each character is
transmitted as a 8-bit byte with the high-order bit cleared to
zero.
November 1981 RFC 788
Simple Mail Transfer Protocol
APPENDIX C
NITS
The Network Independent Transport Service [6] may be used.
Connection Establishment
The SMTP transmission channel is established via NITS between
the sender process and receiver process. The sender process
executes the CONNECT primitive, and the waiting receiver
process executes the ACCEPT primitive.
Data Transfer
The NITS connection supports the transmission of 8-bit bytes.
The SMTP data is 7-bit ASCII characters. Each character is
transmitted as a 8-bit byte with the high-order bit cleared to
zero.
RFC 788 November 1981
Simple Mail Transfer Protocol
APPENDIX D
X.25 Transport service
It may be possible to use the X.25 service [7] as provided by the
Public Data Networks directly, but there are indications that it
is too error prone to qualify as a reliable channel. It is
suggested that a reliable end-to-end protocol such as TCP be used
on top of X.25 connections.
November 1981 RFC 788
Simple Mail Transfer Protocol
APPENDIX E
Theory of Reply Codes
The three digits of the reply each have a special significance.
The first digit denotes whether the response is good, bad or
incomplete. An unsophisticated sender-SMTP will be able to
determine its next action (proceed as planned, redo, retrench,
etc.) by simply examining this first digit. A sender-SMTP that
wants to know approximately what kind of error occurred (e.g.,
mail system error, command syntax error) may examine the second
digit, reserving the third digit for the finest gradation of
information.
There are five values for the first digit of the reply code:
1yz Positive Preliminary reply
The command has been accepted, but the requested action
is being held in abeyance, pending confirmation of the
information in this reply. The sender-SMTP should send
another command specifying whether to continue or abort
the action.
[Note: SMTP does not have any commands that allow this
type of reply, and so does not have the continue or
abort commands.]
2yz Positive Completion reply
The requested action has been successfully completed. A
new request may be initiated.
3yz Positive Intermediate reply
The command has been accepted, but the requested action
is being held in abeyance, pending receipt of further
information. The sender-SMTP should send another command
specifying this information. This reply is used in
command sequence groups.
4yz Transient Negative Completion reply
The command was not accepted and the requested action did
not occur. However, the error condition is temporary and
the action may be requested again. The sender should
RFC 788 November 1981
Simple Mail Transfer Protocol
return to the beginning of the command sequence (if any).
It is difficult to assign a meaning to "transient" when
two different sites (receiver- and sender- SMTPs) must
agree on the interpretation. Each reply in this category
might have a different time value, but the sender-SMTP is
encouraged to try again. A rule of thumb to determine if
a reply fits into the 4yz or the 5yz category (see below)
is that replies are 4yz if they can be repeated without
any change in command form or in properties of the sender
or receiver. (E.g., the command is repeated identically
and the receiver does not put up a new implementation.)
5yz Permanent Negative Completion reply
The command was not accepted and the requested action did
not occur. The sender-SMTP is discouraged from repeating
the exact request (in the same sequence). Even some
"permanent" error conditions can be corrected, so the
human user may want to direct the sender-SMTP to
reinitiate the command sequence by direct action at some
point in the future (e.g., after the spelling has been
changed, or the user has altered the account status).
The second digit encodes responses in specific categories:
x0z Syntax -- These replies refer to syntax errors,
syntactically correct commands that don't fit any
functional category, and unimplemented or superfluous
commands.
x1z Information -- These are replies to requests for
information, such as status or help.
x2z Connections -- These are replies referring to the
transmission channel.
x3z Unspecified as yet.
x4z Unspecified as yet.
x5z Mail system -- These replies indicate the status of
the receiver mail system vis-a-vis the requested
transfer or other mail system action.
The third digit gives a finer gradation of meaning in each
category specified by the second digit. The list of replies
November 1981 RFC 788
Simple Mail Transfer Protocol
illustrates this. Each reply text is recommended rather than
mandatory, and may even change according to the command with
which it is associated. On the other hand, the reply codes
must strictly follow the specifications in this section.
Receiver implementations should not invent new codes for
slightly different situations from the ones described here, but
rather adapt codes already defined.
For example, a command such as NOOP whose successful execution
does not offer the sender-SMTP any new information will return
a 250 reply. The response is 502 when the command requests an
unimplemented non-site-specific action. A refinement of that
is the 504 reply for a command that is implemented, but that
requests an unimplemented parameter.
The reply text may be longer than a single line; in these cases
the complete text must be marked so the sender-SMTP knows when it
can stop reading the reply. This requires a special format to
indicate a multiple line reply.
The format for multi-line replies requires that every line,
except the last, begin with the reply code, followed
immediately by a hyphen, "-" (also known as minus), followed by
text. The last line will begin with the reply code, followed
immediately by <SP>, optionally some text, and <CRLF>.
For example:
123-First line
123-Second line
123-234 text beginning with numbers
123 The last line
The sender-SMTP then simply needs to search for the reply code
followed by <SP> at the beginning of a line, and ignore all
preceding lines.
RFC 788 November 1981
Simple Mail Transfer Protocol
APPENDIX F
Scenarios
This section presents complete scenarios of several types of SMTP
sessions.
A Typical SMTP Transaction Scenario
This SMTP example shows mail sent by Smith at host USC-ISIF, to
Jones, Green, and Brown at host BBN-UNIX. Here we assume that
host USC-ISIF contacts host BBN-UNIX directly. The mail is
accepted for Jones and Brown. Green does not have a mailbox at
host BBN-UNIX.
-------------------------------------------------------------
R: 220 BBN-UNIX Simple Mail Transfer Service Ready
S: HELO USC-ISIF
R: 250 BBN-UNIX
S: MAIL FROM:<Smith@USC-ISIF>
R: 250 OK
S: RCPT TO:<Jones@BBN-UNIX>
R: 250 OK
S: RCPT TO:<Green@BBN-UNIX>
R: 550 No such user here
S: RCPT TO:<Brown@BBN-UNIX>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 BBN-UNIX Service closing transmission channel
Scenario 1
-------------------------------------------------------------
November 1981 RFC 788
Simple Mail Transfer Protocol
Aborted SMTP Transaction Scenario
-------------------------------------------------------------
R: 220 MIT-Multics Simple Mail Transfer Service Ready
S: HELO ISI-VAXA
R: 250 MIT-Multics
S: MAIL FROM:<Smith@ISI-VAXA>
R: 250 OK
S: RCPT TO:<Jones@MIT-Multics>
R: 250 OK
S: RCPT TO:<Green@MIT-Multics>
R: 550 No such user here
S: RSET
R: 250 OK
S: QUIT
R: 221 MIT-Multics Service closing transmission channel
Scenario 2
-------------------------------------------------------------
RFC 788 November 1981
Simple Mail Transfer Protocol
Relayed Mail Scenario
-------------------------------------------------------------
Step 1 -- Source Host to Relay Host
R: 220 USC-ISIE Simple Mail Transfer Service Ready
S: HELO MIT-AI
R: 250 USC-ISIE
S: MAIL FROM:<JQP@MIT-AI>
R: 250 OK
S: RCPT TO:<@ISIE,Jones@BBN-VAX>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Date: 2-Nov-81 22:33:44
S: From: John Q. Public <JQP at MIT-AI>
S: Subject: The Next Meeting of the Board
S: To: Jones at BBN-Vax
S:
S: Bill:
S: The next meeting of the board of directors will be
S: on Tuesday.
S: John.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIE Service closing transmission channel
November 1981 RFC 788
Simple Mail Transfer Protocol
Step 2 -- Relay Host to Destination Host
R: 220 BBN-VAX Simple Mail Transfer Service Ready
S: HELO USC-ISIE
R: 250 BBN-VAX
S: MAIL FROM:<@ISIE,JQP@MIT-AI>
R: 250 OK
S: RCPT TO:<Jones@BBN-VAX>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Mail-From: NCP host MIT-AI received by USC-ISIE at
2-Nov-81 22:40:10
S: Date: 2-Nov-81 22:33:44
S: From: John Q. Public <JQP at MIT-AI>
S: Subject: The Next Meeting of the Board
S: To: Jones at BBN-Vax
S:
S: Bill:
S: The next meeting of the board of directors will be
S: on Tuesday.
S: John.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIE Service closing transmission channel
Scenario 3
-------------------------------------------------------------
RFC 788 November 1981
Simple Mail Transfer Protocol
Verifying and Sending Scenario
-------------------------------------------------------------
R: 220 SU-SCORE Simple Mail Transfer Service Ready
S: HELO MIT-MC
R: 250 SU-SCORE
S: VRFY Crispin
R: 250 Mark Crispin <Admin.MRC@SU-SCORE>
S: SEND FROM:<EAK@MIT-MC>
R: 250 OK
S: RCPT TO:<Admin.MRC@SU-SCORE>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 SU-SCORE Service closing transmission channel
Scenario 4
-------------------------------------------------------------
November 1981 RFC 788
Simple Mail Transfer Protocol
Sending and Mailing Scenarios
First the user's name is verified, then an attempt is made to
send to the user's terminal. When that fails, the messages is
mailed to the user's mailbox.
-------------------------------------------------------------
R: 220 SU-SCORE Simple Mail Transfer Service Ready
S: HELO MIT-MC
R: 250 SU-SCORE
S: VRFY Crispin
R: 250 Mark Crispin <Admin.MRC@SU-SCORE>
S: SEND FROM:<EAK@MIT-MC>
R: 250 OK
S: RCPT TO:<Admin.MRC@SU-SCORE>
R: 450 User not active now
S: RSET
R: 250 OK
S: MAIL FROM:<EAK@MIT-MC>
R: 250 OK
S: RCPT TO:<Admin.MRC@SU-SCORE>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 SU-SCORE Service closing transmission channel
Scenario 5
-------------------------------------------------------------
RFC 788 November 1981
Simple Mail Transfer Protocol
Doing the preceding scenario more efficiently.
-------------------------------------------------------------
R: 220 SU-SCORE Simple Mail Transfer Service Ready
S: HELO MIT-MC
R: 250 SU-SCORE
S: VRFY Crispin
R: 250 Mark Crispin <Admin.MRC@SU-SCORE>
S: SOML FROM:<EAK@MIT-MC>
R: 250 OK
S: RCPT TO:<Admin.MRC@SU-SCORE>
R: 250 User not active now, so will do mail.
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 SU-SCORE Service closing transmission channel
Scenario 6
-------------------------------------------------------------
November 1981 RFC 788
Simple Mail Transfer Protocol
Mailing List Scenario
First each of two mailing lists are expanded in separate sessions
with different hosts. Then the message is sent to everyone that
appeared on either list (but no duplicates) via a relay host.
-------------------------------------------------------------
Step 1 -- Expanding the First List
R: 220 MIT-AI Simple Mail Transfer Service Ready
S: HELO SU-SCORE
R: 250 MIT-AI
S: EXPN Example-People
R: 250-<ABC@MIT-MC>
R: 250-Fred Fonebone <Fonebone@ISIQ>
R: 250-Xenon Y. Zither <XYZ@MIT-AI>
R: 250-Quincy Smith <@ISIF,Q-Smith@ISI-VAXA>
R: 250-<joe@foo-unix>
R: 250 <xyz@bar-unix>
S: QUIT
R: 221 MIT-AI Service closing transmission channel
RFC 788 November 1981
Simple Mail Transfer Protocol
Step 2 -- Expanding the Second List
R: 220 MIT-MC Simple Mail Transfer Service Ready
S: HELO SU-SCORE
R: 250 MIT-MC
S: EXPN Interested-Parties
R: 250-Al Calico <ABC@MIT-MC>
R: 250-<XYZ@MIT-AI>
R: 250-Quincy Smith <@ISIF,Q-Smith@ISI-VAXA>
R: 250-<fred@BBN-UNIX>
R: 250 <xyz@bar-unix>
S: QUIT
R: 221 MIT-MC Service closing transmission channel
November 1981 RFC 788
Simple Mail Transfer Protocol
Step 3 -- Mailing to All via a Relay Host
R: 220 USC-ISIE Simple Mail Transfer Service Ready
S: HELO SU-SCORE
R: 250 USC-ISIE
S: MAIL FROM:<Account.Person@SU-SCORE>
R: 250 OK
S: RCPT TO:<@ISIE,ABC@MIT-MC>
R: 250 OK
S: RCPT TO:<@ISIE,Fonebone@ISIQ>
R: 250 OK
S: RCPT TO:<@ISIE,XYZ@MIT-AI>
R: 250 OK
S: RCPT TO:<@ISIE,@ISIF,Q-Smith@ISI-VAXA>
R: 250 OK
S: RCPT TO:<@ISIE,joe@FOO-UNIX>
R: 250 OK
S: RCPT TO:<@ISIE,xyz@BAR-UNIX>
R: 250 OK
S: RCPT TO:<@ISIE,fred@BBN-UNIX>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIE Service closing transmission channel
Scenario 7
-------------------------------------------------------------
RFC 788 November 1981
Simple Mail Transfer Protocol
Forwarding Scenarios
-------------------------------------------------------------
R: 220 USC-ISIF Simple Mail Transfer Service Ready
S: HELO LBL-UNIX
R: 250 USC-ISIF
S: MAIL FROM:<mo@LBL-UNIX>
R: 250 OK
S: RCPT TO:<fred@USC-ISIF>
R: 251 User not local; will forward to <Jones@USC-ISIA>
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIF Service closing transmission channel
Scenario 8
-------------------------------------------------------------
November 1981 RFC 788
Simple Mail Transfer Protocol
-------------------------------------------------------------
Step 1 -- Trying the Mailbox at the First Host
R: 220 USC-ISIF Simple Mail Transfer Service Ready
S: HELO LBL-UNIX
R: 250 USC-ISIF
S: MAIL FROM:<mo@LBL-UNIX>
R: 250 OK
S: RCPT TO:<fred@USC-ISIF>
R: 251 User not local; will forward to <Jones@USC-ISIA>
S: RSET
R: 250 OK
S: QUIT
R: 221 USC-ISIF Service closing transmission channel
Step 2 -- Delivering the Mail at the Second Host
R: 220 USC-ISIA Simple Mail Transfer Service Ready
S: HELO LBL-UNIX
R: 250 USC-ISIA
S: MAIL FROM:<mo@LBL-UNIX>
R: 250 OK
S: RCPT TO:<Jones@USC-ISIA>
R: OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIA Service closing transmission channel
Scenario 9
-------------------------------------------------------------
RFC 788 November 1981
Simple Mail Transfer Protocol
Too Many Recipients Scenario
-------------------------------------------------------------
R: 220 BERKELEY Simple Mail Transfer Service Ready
S: HELO USC-ISIF
R: 250 BERKELEY
S: MAIL FROM:<Postel@USC-ISIF>
R: 250 OK
S: RCPT TO:<fabry@BERKELEY>
R: 250 OK
S: RCPT TO:<eric@BERKELEY>
R: 552 Recipient storage full, try again in another transaction
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: MAIL FROM:<Postel@USC-ISIF>
R: 250 OK
S: RCPT TO:<eric@BERKELEY>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 BERKELEY Service closing transmission channel
Scenario 10
-------------------------------------------------------------
November 1981 RFC 788
Simple Mail Transfer Protocol
GLOSSARY
ASCII
American Standard Code for Information Interchange [1].
command
A request for a mail service action sent by the sender-SMTP to the
receiver-SMTP.
end of mail data indication
A special sequence of characters that indicates the end of the
mail data. In particular, the five characters carriage return,
line feed, period, carriage return, line feed, in that order.
host
A computer in the internetwork environment on which mailboxes or
SMTP processes reside.
line
A line of text ending with a <CRLF>.
mail data
A sequence of ASCII characters of arbitrary length, which conforms
to the standard set in the Standard for the Format of ARPA Network
Text Messages (RFC 733 [2]).
mailbox
A character string (address) which identifies a user to whom mail
is to be sent. Mailbox normally consists of the host and user
specifications. The standard mailbox naming convention is defined
to be "user@host". Additionally, the "container" in which mail is
stored.
receiver-SMTP process
A process which transfers mail in cooperation with a sender-SMTP
process. It waits for a connection to be established via the
transport service. It receives SMTP commands from the
sender-SMTP, sends replies, and performs the specified operations.
RFC 788 November 1981
Simple Mail Transfer Protocol
reply
A reply is an acknowledgment (positive or negative) sent from
receiver to sender via the transmission channel in response to a
SMTP command. The general form of a reply is a completion code
(including error codes) followed by a text string. The codes are
for use by programs and the text is usually intended for human
users.
sender-SMTP process
A process which transfers mail in cooperation with a receiver-SMTP
process. A local language may be used in the user interface
command/reply dialogue. The sender-SMTP initiates the transport
service connection. It initiates SMTP commands, receives replies,
and governs the transfer of mail.
session
The set of exchanges that occur while the transmission channel is
open.
transaction
The set of exchanges required for one message to be transmitted
for one or more recipients.
transmission channel
A full-duplex communication path between a sender-SMTP and a
receiver-SMTP for the exchange of commands, replies, and mail
text.
transport service
Any reliable stream-oriented data communication services. For
example, NCP, TCP, NITS.
user
A human being (or a process on behalf of a human being) wishing to
obtain mail transfer service. In addition, a recipient of
computer mail.
November 1981 RFC 788
Simple Mail Transfer Protocol
word
A sequence of printing characters.
<CRLF>
The characters carriage return and line feed (in that order).
<SP>
The space character.
RFC 788 November 1981
Simple Mail Transfer Protocol
REFERENCES
[1] ASCII
ASCII, "USA Code for Information Interchange", United States of
America Standards Institute, X3.4, 1968. Also in: Feinler, E.
and J. Postel, eds., "ARPANET Protocol Handbook", NIC 7104, for
the Defense Communications Agency by SRI International, Menlo
Park, California, Revised January 1978.
[2] RFC 733
Crocker, D., J. Vittal, K. Pogran, and D. Henderson, "Standard for
the Format of ARPA Network Text Messages," RFC 733, NIC 41952,
November 1977. Also in: Feinler, E. and J. Postel, eds.,
"ARPANET Protocol Handbook", NIC 7104, for the Defense
Communications Agency by SRI International, Menlo Park,
California, Revised January 1978.
[3] TCP
Postel, J., ed., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", RFC 793, USC/Information Sciences
Institute, September 1981.
[4] NCP
McKenzie,A., "Host/Host Protocol for the ARPA Network", NIC 8246,
January 1972. Also in: Feinler, E. and J. Postel, eds., "ARPANET
Protocol Handbook", NIC 7104, for the Defense Communications
Agency by SRI International, Menlo Park, California, Revised
January 1978.
[5] Initial Connection Protocol
Postel, J., "Official Initial Connection Protocol", NIC 7101,
11 June 1971. Also in: Feinler, E. and J. Postel, eds., "ARPANET
Protocol Handbook", NIC 7104, for the Defense Communications
Agency by SRI International, Menlo Park, California, Revised
January 1978.
[6] NITS
PSS/SG3, "A Network Independent Transport Service", Study Group 3,
The Post Office PSS Users Group, February 1980. Available from
the DCPU, National Physical Laboratory, Teddington, UK.
November 1981 RFC 788
Simple Mail Transfer Protocol
[7] X.25
CCITT, "Recommendation X.25 - Interface Between Data Terminal
Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for
Terminals Operating in the Packet Mode on Public Data Networks,"
CCITT Orange Book, Vol. VIII.2, International Telephone and
Telegraph Consultative Committee, Geneva, 1976.

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